Alison Carey, 55, was arrested on prostitution charges earlier week at a low budget motel in upstate New York. She has pleaded not guilty.
Cops say she placed an online ad soliciting sex that borrowed lines from her sister's 1996 hit, "Fantasy."
See photos of Mariah Carey through the years:
Mariah Carey through the years
Mariah Carey through the years
NEW YORK, NY - FEBRUARY 12: Singer Mariah Carey and Nick Cannon attend MTV First: Mariah Carey's 'You're Mine (Eternal)' music video world premiere at MTV Studios on February 12, 2014 in New York City. (Photo by Larry Busacca/Getty Images)
LOS ANGELES, CA - JANUARY 18: Mariah Carey and Nick Cannon (R) arrive at the 20th Annual Screen Actors Guild Awards at the Shrine Auditorium on January 18, 2014 in Los Angeles, California. (Photo by Dan MacMedan/WireImage)
NEW YORK, NY - AUGUST 05: Nick Cannon (L) and Mariah Carey attend Lee Daniels' 'The Butler' New York premiere, hosted by TWC, DeLeon Tequila and Samsung Galaxy on August 5, 2013 in New York City. (Photo by Kevin Mazur/Getty Images for Samsung)
HOLLYWOOD, CA - NOVEMBER 17: TeenNick Chairman and HALO Awards host Nick Cannon and singer Mariah Carey attend Nickelodeon's 2012 TeenNick HALO Awards at Hollywood Palladium on November 17, 2012 in Hollywood, California. The show premieres on Monday, November 19th, 8:00p.m. (ET) on Nick at Nite. (Photo by Charley Gallay/Getty Images For Nickelodeon)
NEW YORK, NY - DECEMBER 15: Mariah Carey leaves the Beacon Theater on December 15, 2014 in New York City. (Photo by James Devaney/GC Images)
LOS ANGELES, CA - MARCH 14: Mariah Carey (L) and actor Jussie Smollett attend the Human Rights Campaign Los Angeles Gala 2015 at JW Marriott Los Angeles at L.A. LIVE on March 14, 2015 in Los Angeles, California. (Photo by Jason Kempin/Getty Images for Human Rights Campaign)
NEW YORK, NY - DECEMBER 03: Mariah Carey performs at the 82nd Annual Rockefeller Christmas Tree Lighting Ceremony at Rockefeller Center on December 3, 2014 in New York City. (Photo by James Devaney/WireImage)
NEW YORK, NY - DECEMBER 03: Singer Mariah Carey performs at the 82nd Annual Rockefeller Center Christmas Tree Lighting on December 3, 2014 in New York City. (Photo by Paul Zimmerman/WireImage)
SHANGHAI, CHINA - OCTOBER 19: Singer Mariah Carey performs live during her 2014 China Tour on October 19, 2014 in Shanghai, China. (Photo by ChinaFotoPress/ChinaFotoPress via Getty Images)
NEW YORK NY - AUGUST 26: Mariah Carey sighting on August 26, 2014 in New York City. (Photo by Josiah Kamau/BuzzFoto/FilmMagic)
HOLLYWOOD, CA - JULY 23: Singer Mariah Carey and actor Dwayne Johnson attend the after party for the premiere of Paramount Pictures' 'Hercules' at the Roosevelt Hotel on July 23, 2014 in Hollywood, California. (Photo by Alberto E. Rodriguez/WireImage)
CAP D'ANTIBES, FRANCE - JUNE 17: Mariah Carey performs during Clear Channel Media And Entertainment And MediaLink Dinner at Hotel du Cap-Eden-Roc on June 17, 2014 in Cap d'Antibes, France. (Photo by Tony Barson/Getty Images for Clear Channel)
NEW YORK, NY - JUNE 09: Singer Mariah Carey announces the launch of her Go N'Syde bottle 'Butterfly' at the Saint Regis Hotel on June 9, 2014 in New York City. (Photo by Andrew H. Walker/Getty Images)
NEW YORK, NY - MAY 29: Singer Mariah Carey attends the 2014 Fresh Air Fund Honoring Our American Hero at Pier Sixty at Chelsea Piers on May 29, 2014 in New York City. (Photo by Monica Schipper/FilmMagic)
Mariah Carey performs during the World Music Awards at Monte Carlo Sporting Club, on May 27, 2014 in Monaco. (Photo credit: VALERY HACHE/AFP/Getty Images)
TODAY -- Pictured: Mariah Carey appears on NBC News' 'Today' show -- (Photo by: Peter Kramer/NBC/NBC NewsWire via Getty Images)
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The arrest is the latest crisis for Alison, who in her younger days wanted to be a singer just like Mariah, but she ended up battling drugs and falling on hard times.
Morgan said: "I would hope Mariah could find it in her heart to forgive Alison her transgressions and step up, create a trust, let's make sure Alison's needs are met."
Morgan and Mariah were once very close. In fact, Morgan — a model and music producer — claims he was instrumental in Mariah becoming a superstar.
Mariah's publicist says she has done a lot over the years to give Alison a helping hand.
"Through the years, Mariah has spent hundreds of thousands of dollars supporting Alison and her children," the statement said.
Morgan says: "I will always be puzzled why Alison, a heroin addict, has always been given large sums of money from Mariah her whole life. You can't put large sums of cash into the hands of a drug addict."
Incredibly, Morgan fears retaliation from his famous sister and her fans for speaking out.
"It's about protecting this fiction she has created, this image she has created, it's about keeping the darkest secrets covered up," he said.
It is believed Mariah has not spoken to her sister since 1994.